JP2016208310A - Optical transmission system and optical transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a variation in a gain ripple accompanying a variation in wavelength arrangement of a WDM signal.SOLUTION: An optical transmission device comprises: a wavelength arrangement detection unit for detecting wavelength arrangement showing arrangement of optical signals multiplexed into wavelength multiplexed optical signals; a power adjustor for adjusting power of the optical signals multiplexed into the wavelength multiplexed optical signals; an optical amplifier for amplifying the wavelength multiplexed optical signals output from the power adjustor; a power control unit for generating a power control signal for controlling the power adjustor so that the wavelength multiplexed optical signals have a designated wavelength characteristic; and a correction value generation unit for generating a correction value for correcting the power control signal on the basis of the wavelength arrangement. The power control unit corrects the power control signal by using the correction value. The power adjustor adjusts the power of the optical signals multiplexed into the wavelength multiplexed optical signals, according to the corrected power control signal.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an optical transmission apparatus and an optical transmission system that transmit wavelength division multiplexed optical signals.

  In recent years, WDM transmission systems using wavelength division multiplexing (WDM) have become widespread. The WDM can multiplex and transmit a plurality of optical signals having different wavelengths. In the WDM transmission system, an optical add / drop multiplexer (ROADM) is provided at each node. ROADM can branch an optical signal having a desired wavelength from a WDM signal, and can insert an optical signal into an empty channel of the WDM signal.

  In the WDM transmission system, an optical amplifier is provided at each node in order to compensate for loss in the transmission line fiber and the ROADM. As an optical amplifier that amplifies the WDM signal, for example, a rare earth doped fiber amplifier (EDFA) is used.

  The wavelength characteristics of the optical gain and optical loss for the WDM signal depend on the wavelength arrangement of the WDM signal. Therefore, the WDM transmission system has a function of adjusting the optical power of each wavelength channel based on the wavelength arrangement of the WDM signal. This function is realized by, for example, an optical channel monitor (OCM) that detects the optical power of each wavelength channel and a wavelength selective switch (WSS) that adjusts the optical power of each wavelength channel. In this case, the optical power of each wavelength channel is controlled so that the power of the optical signal arriving at the receiving node is maintained within the receivable power range of the optical receiver.

  However, detection of optical power by the optical channel monitor and / or adjustment of optical power by the wavelength selective switch requires several hundred milliseconds to several seconds. For this reason, when the wavelength arrangement of the WDM signal changes, the optical power of each wavelength channel may fluctuate greatly temporarily. And if the power of the optical signal arriving at the receiving node is out of the receivable power range of the optical receiver, a signal error occurs.

  This problem occurs when the change in the optical gain wavelength characteristic due to the change in the wavelength arrangement of the WDM signal is large. When the gain ripple with respect to the WDM signal in the steady state is large, the change in the optical gain wavelength characteristic due to the change in the wavelength arrangement of the WDM signal becomes large. That is, if the gain ripple for the WDM signal in the steady state can be reduced, the change in the optical gain wavelength characteristic due to the change in the wavelength arrangement of the WDM signal is reduced. In this case, even if the wavelength arrangement of the WDM signal changes, fluctuations in the optical power of each wavelength channel are suppressed, and signal errors in the optical receiver are also suppressed. The gain ripple represents a change in optical gain with respect to the wavelength.

  One of the factors causing EDFA gain ripple is a spectrum hole burning (SHB) phenomenon. Spectral hole burning occurs when an optical signal passes through an EDFA. Specifically, when the optical signal passes through the EDFA, the gain of the wavelength of the optical signal and the wavelength in the vicinity thereof are reduced.

  Therefore, if the gain ripple caused by the spectral hole burning phenomenon is reduced, the fluctuation of the optical power of each wavelength channel is suppressed even when the wavelength arrangement of the WDM signal is changed.

  As one configuration for suppressing a change in gain ripple, an optical transmission device that inserts pseudo light into a wavelength band not used for signal light has been proposed (for example, Patent Document 1). As a related technique, a method for accurately measuring the intensity of an optical signal in a WDM communication system has been proposed (for example, Patent Document 2). Further, Non-Patent Documents 1 and 2 below describe EDFA modeling and spectral hole burning phenomenon.

JP 2008-091995 A JP 2008-139073 A

C. Randy Giles and Emmanuel Desurvire, Modeling Erbium-Doped Fiber Amplifiers, Journal of Lightwave Technology, Vol. 9, No. 2. 271-283 (1991) Maxim Bolshtyansky, Spectral Hole Burning in Erbium-Doped Fiber Amplifiers, Journal of Lightwave Technology, Vol. 21, No. 4. 1032-1038 (2003)

  In the prior art, it is difficult to effectively suppress the gain ripple caused by the spectral hole burning phenomenon. For this reason, it is difficult to suppress a change in gain ripple accompanying a change in wavelength arrangement of the WDM signal. In the method of inserting pseudo light into a wavelength band that is not used for signal light, some wavelength channels of the WDM signal may not be used for data transmission.

  An object according to one aspect of the present invention is to suppress a change in gain ripple accompanying a change in wavelength arrangement of a WDM signal.

  An optical transmission device according to one aspect of the present invention is multiplexed with a wavelength arrangement detection unit that detects a wavelength arrangement representing an arrangement of an optical signal multiplexed with a wavelength multiplexed optical signal, and the wavelength multiplexed optical signal. A power adjuster for adjusting the power of the optical signal; an optical amplifier for amplifying the wavelength multiplexed optical signal output from the power adjuster; and the power adjuster so that the wavelength multiplexed optical signal has a specified wavelength characteristic. And a correction value generation unit that generates a correction value for correcting the power control signal based on the wavelength arrangement. The power control unit corrects the power control signal with the correction value. The power adjuster adjusts the power of the optical signal multiplexed on the wavelength-multiplexed optical signal according to the corrected power control signal.

  According to the above-described aspect, the change in the gain ripple accompanying the change in the wavelength arrangement of the WDM signal is suppressed.

It is a figure which shows an example of a WDM transmission system, and the power of a received optical signal. It is a figure which shows the spectrum of the spectrum hole burning phenomenon resulting from one optical signal. It is FIG. (1) explaining the spectrum hole burning phenomenon resulting from a some optical signal. It is FIG. (2) explaining the spectrum hole burning phenomenon resulting from a some optical signal. It is a figure which shows an example of an optical transmission system. It is a figure which shows an example of the optical transmission apparatus and optical transmission system of 1st Embodiment. It is a figure which shows the example of wavelength arrangement | positioning of a WDM signal. It is a figure which shows an example of a power density / correction value conversion table. It is a flowchart which shows an example of a process of a controller. It is a figure which shows the state and gain ripple of a WDM signal when correction | amendment is not performed. It is a figure which shows the state and gain ripple of a WDM signal when correction | amendment is performed. It is a figure explaining flattening of a gain. It is a figure which shows the effect by correction | amendment. It is a figure which shows an example of the optical transmission system of 2nd Embodiment. It is a figure which shows an example of the optical transmission system of 3rd Embodiment. It is a figure which shows an example of the optical transmission system of 4th Embodiment. It is a figure which shows an example of the optical transmission system of 5th Embodiment.

  The optical transmission device and the optical transmission system according to the embodiment of the present invention suppress the gain ripple caused by the spectral hole burning phenomenon. Therefore, before describing the configuration and operation of the optical transmission apparatus, gain ripple caused by the spectral hole burning phenomenon will be briefly described.

  In the following description, it is assumed that 88 wavelength channels (ch1 to ch88) can be multiplexed into a WDM signal. The optical signal arranged in the wavelength channel chx may be referred to as “optical signal chx”.

  Each node of the optical transmission system that transmits a WDM signal is provided with a ROADM as shown in FIG. The transmitter Tx1 transmits optical signals ch11 to ch19 to the receiver Rx. The transmitter Tx2 transmits optical signals ch1 and ch39 to the receiver Rx. The optical signals ch1, ch11-19, and ch39 are multiplexed with the WDM signal and transmitted.

  The ROADM provided in each node can adjust the power of each optical signal multiplexed in the WDM signal. As an example, ROADM equalizes the power of a plurality of optical signals multiplexed on a WDM signal. “Equalization” means that the power of each optical signal is controlled so that the power of a plurality of optical signals is substantially the same or substantially the same. The ROADM also includes an optical amplifier that amplifies the WDM signal. In this embodiment, the optical amplifier is realized by an EDFA. Then, as shown in FIG. 1B, the ROADM controls the power of the WDM signal using the EDFA so that the received optical power at the receiver Rx is maintained within the receivable range.

  Spectral hole burning occurs when an optical signal passes through an EDFA. Specifically, when the optical signal passes through the EDFA, the gain of the wavelength of the optical signal and the wavelength in the vicinity thereof are reduced.

  FIG. 2 shows the spectrum of the spectral hole burning phenomenon. FIG. 2A shows the gain spectrum when the optical signal ch1 (λ1 = 1528.77 nm) passes through the EDFA. When the optical signal ch1 passes through the EDFA, as shown in FIG. 2A, the gain in λ1 and the wavelength region in the vicinity thereof decreases. Similarly, when the optical signal ch11 (λ11 = 1153.68 nm) passes through the EDFA, as shown in FIG. 2B, the gain of λ11 and the wavelength region in the vicinity thereof decreases. When the optical signal ch19 (λ19 = 1535.82 nm) passes through the EDFA, as shown in FIG. 2C, the gain in λ19 and the wavelength region in the vicinity thereof decreases. When the optical signal ch39 (λ39 = 1543.87 nm) passes through the EDFA, as shown in FIG. 2D, the gain of λ39 and the wavelength region in the vicinity thereof decreases. Note that the “hole” in the spectrum corresponds to a state in which the gain is lower than in other wavelength regions.

  The spectral hole burning phenomenon when a plurality of optical signals pass through the EDFA can be approximately expressed by a weighted average of the spectral hole burning phenomenon of each optical signal. The weight of the weighted average depends on the power of each optical signal. That is, the weight of an optical signal with high power is large, and the weight of an optical signal with low power is small. FIG. 3A shows the spectrum of the spectral hole burning phenomenon when optical signals ch1, ch11 to ch19, and ch39 are transmitted. FIG. 3B shows a weighted average of a plurality of spectra shown in FIG. Thus, when a plurality of optical signals are transmitted, “holes” are likely to be formed in a wavelength region (about 1530 to 1540 nm) where a plurality of optical signals (ch11 to ch19) are densely arranged.

  The EDFA provided in each node is controlled in, for example, an AGC (Automatic Gain Control) mode. In AGC, the average gain is held at a specified target value. In this case, for example, the excitation power of the EDFA is controlled so that the ratio of the total optical power of the input WDM signal of the EDFA and the total optical power of the output WDM signal of the EDFA approaches the target value.

  FIG. 3C shows gain characteristics of the EDFA when optical signals ch1, ch11 to ch19, and ch39 are transmitted. The dashed line represents the average gain held in AGC. As shown in FIG. 3C, the gain in the wavelength region where the arrangement of optical signals is “dense” (that is, the wavelength region in which ch11 to ch19 are arranged) is lower than the average gain held by the AGC. It has become. On the other hand, the gain in the wavelength region where the arrangement of optical signals is “sparse” (that is, the wavelength region in which ch1 and ch39 are arranged) is higher than the average gain held by the AGC. As described above, the fluctuation in gain (that is, gain ripple) due to the spectral hole burning phenomenon depends on the wavelength arrangement of the WDM signal. Specifically, the gain in the wavelength region where the power density is high tends to be lower than the average gain due to the spectral hole burning phenomenon, and the gain in the wavelength region where the power density is low is attributed to the spectral hole burning phenomenon. Tend to be higher than the average gain.

Here, a change in the gain ripple when the wavelength arrangement of the WDM signal is changed will be considered. As an example, consider a case where the optical signals ch11 to ch19 stop in a steady state where the optical signals ch1, ch11 to ch19, and ch39 are transmitted. Such a case occurs, for example, in the following case in the optical transmission system shown in FIG.
(1) The transmitter Tx1 stops the transmission of the optical signals ch11 to ch19.
(2) The destinations of the optical signals ch11 to ch19 are changed.
(3) The optical fiber between node A and node B is cut.

  The spectral hole burning phenomenon after the optical signals ch11 to ch19 are stopped is approximately expressed by a weighted average of the spectral hole burning phenomenon of the optical signal ch1 and the spectral hole burning phenomenon of the optical signal ch39. FIG. 4A shows the spectrum of the spectral hole burning phenomenon when only the optical signals ch1 and ch39 are transmitted. FIG. 4B shows a weighted average of the spectrum shown in FIG.

  When the optical signals ch11 to ch19 are stopped in the steady state in which the optical signals ch1, ch11 to ch19, and ch39 are transmitted, the gain of the EDFA changes from the state shown in FIG. 3C to the state shown in FIG. Transition. At this time, the gain for the optical signals ch1 and ch39 remaining in the WDM signal changes greatly. In particular, in this simulation, the gain with respect to the optical signal ch1 is greatly reduced. As a result, the power of the optical signal ch1 greatly decreases immediately after the optical signals ch11 to ch19 are stopped.

  A change in the gain of the EDFA due to a change in the wavelength arrangement of the WDM signal occurs at each node. When the WDM signal is transmitted via a plurality of nodes (that is, when the transmission path of the WDM signal is composed of a plurality of OMSs (Optical Multiplex Sections)), the gain fluctuation generated at each node is accumulated. . As a result, immediately after the wavelength arrangement of the WDM signal changes, the power of the optical signal remaining in the WDM signal arriving at the receiver Rx varies greatly. In some cases, the power of the received optical signal falls outside the receivable power range of the receiver Rx. In the example shown in FIG. 1C, the reception power of the optical signal ch1 in the receiver Rx is lower than the lowest receivable level of the receiver Rx.

  The above problem is likely to occur when the gain ripple in the steady state is large. In other words, if the gain ripple in the steady state is reduced, the change in the optical gain wavelength characteristic due to the change in the wavelength arrangement of the WDM signal is reduced. Therefore, the optical transmission apparatus according to the embodiment of the present invention has a function of reducing the gain ripple in the steady state. That is, the optical transmission apparatus according to the embodiment of the present invention has a function of flattening the gain with respect to the wavelength in a steady state.

The gain G of the EDFA is expressed by the following model, for example.
λ represents a wavelength. z represents a position in the optical fiber. S represents the gain of the EDFA when the spectral hole burning phenomenon is not taken into consideration. H represents a parameter that does not depend on optical power and gain. M represents the weight of the weighted average. λj represents the wavelength of the optical signal chj.

  In this model, “S (λj, z) H (λj, λ)” represents a spectral hole burning phenomenon caused by the optical signal chj. That is, when a plurality of optical signals are amplified by the EDFA, the gain of the EDFA can be obtained by calculating a weighted average with M for the spectral hole burning phenomenon caused by each optical signal. Here, the weight M (λj, z) of the optical signal chj depends on the power of the optical signal chj. Therefore, when the power of the optical signal chj changes, the spectrum of the gain of the EDFA also changes.

  FIG. 5 shows an example of an optical transmission system according to an embodiment of the present invention. FIG. 5 shows a part of the optical transmission system.

  Each node of the optical transmission system is provided with an optical transmission device including ROADM. The nodes are connected by a transmission line fiber. That is, a WDM signal transmitted from an optical transmission apparatus provided at a certain node is transmitted via a transmission line fiber and received by an optical transmission apparatus provided at an adjacent node. In the example shown in FIG. 5, the WDM signal transmitted from the optical transmission device 1A is transmitted through the transmission line fiber 2 and received by the optical transmission device 1B. In the following description, it is assumed that the configuration of the optical transmission device provided in each node is substantially the same.

  The optical transmission device 1 (1A, 1B) includes a preamplifier 11, a wavelength selective switch (WSS) 12, an optical splitter 13, an optical channel monitor (OCM) 14, and a postamplifier 15. The preamplifier 11 amplifies a WDM signal received from another node. For example, the preamplifier 11 amplifies the received WDM signal in the AGC mode. Note that the preamplifier 11 is realized by an EDFA.

  As shown in FIG. 5, the wavelength selective switch 12 includes a duplexer 21, a ROADM 22, a variable optical attenuator (VOA) 23, a multiplexer 24, a controller 25, and a memory 26, and is amplified by the preamplifier 11. The processed WDM signal is processed. However, the optical transmission device 1 may be configured to include a wavelength blocker instead of the wavelength selective switch 12.

  The duplexer 21 separates the WDM signal amplified by the preamplifier 11 for each wavelength. That is, the optical signals of the respective wavelength channels are separated from each other. The ROADM 22 processes the optical signal of each wavelength channel according to the path information. The path information is given from, for example, a user or a network administrator. For example, the ROADM 22 branches the optical signal specified by the path information from the WDM signal. The branched optical signal is guided to a client, for example. In addition, the ROADM 22 can insert an add optical signal into an unused wavelength channel. The add optical signal is generated by, for example, a client. The optical signals output from the ROADM 22 are guided to the corresponding variable optical attenuators 23, respectively.

  The variable optical attenuator 23 can adjust the attenuation amount for each wavelength channel of the WDM signal. For example, in a configuration in which the WDM transmission system transmits the wavelength channels λ1 to λ88, the wavelength selective switch 12 may include 88 variable optical attenuators 23. The variable optical attenuator 23 adjusts the power of the optical signal according to the control signal given from the controller 25. This control signal indicates, for example, an attenuation amount. The multiplexer 24 combines a plurality of optical signals output from the plurality of variable optical attenuators 23 to generate a WDM signal.

  The controller 25 generates a control signal that controls the variable optical attenuator 23. At this time, the controller 25 generates a control signal based on the optical power of each wavelength channel detected by the optical channel monitor 14 so that the WDM signal output from the wavelength selective switch 12 has a specified wavelength characteristic. . As an example, the controller 25 generates a control signal that equalizes the optical power of the WDM signal output from the wavelength selective switch 12. Further, the controller 25 may correct the above correction signal based on the wavelength arrangement of the WDM signal. When the control signal is corrected, the variable optical attenuator 23 adjusts the power of the optical signal according to the corrected control signal.

  The memory 26 stores correction information for correcting the control signal. The correction information, which will be described in detail later, is used to suppress the gain ripple caused by the spectral hole burning phenomenon. Therefore, the controller 25 can correct the control signal with reference to the correction information stored in the memory 26 as necessary.

  The controller 25 provides a function of adjusting the power of each optical signal, for example, by executing a given program. In this case, the controller 25 is realized by a processor. However, the controller 25 may include a hardware circuit. That is, the function of adjusting the power of each optical signal may be realized by a combination of software processing and hardware circuits. The program executed by the processor may be stored in the memory 26.

  The optical splitter 13 branches the WDM signal output from the wavelength selective switch 12 and guides it to the optical channel monitor 14. The optical splitter 13 is realized by an optical coupler, for example. The optical channel monitor 14 detects the power of each optical signal multiplexed in the WDM signal output from the wavelength selective switch 12. That is, the optical power of each wavelength channel is detected by the optical channel monitor 14. Then, OCM information indicating the optical power of each wavelength channel is given to the controller 25.

  The post amplifier 15 amplifies the WDM signal output from the wavelength selective switch 12. In this embodiment, the postamplifier 15 amplifies the WDM signal in the AGC mode. In AGC, the average gain of a WDM signal is held at a specified target value. Note that the post-amplifier 15 is realized by an EDFA.

  In the optical transmission system configured as described above, the operation according to the embodiment of the present invention is realized in each optical transmission apparatus. However, the operation according to the embodiment of the present invention may be realized by two optical transmission apparatuses. In this case, the operation according to the embodiment of the present invention is realized in the OMS including the wavelength selective switch 12 of the transmission side optical transmission apparatus, the optical channel monitor 14, the post 15, and the preamplifier 11 of the reception side optical transmission apparatus. .

<First Embodiment>
FIG. 6 illustrates an example of the optical transmission apparatus and the optical transmission system according to the first embodiment. In the first embodiment, the optical transmission device 1 includes a wavelength arrangement detection unit 31, a power density calculation unit 32, a correction value generation unit 33, and a power control unit 34. Functions of the wavelength arrangement detection unit 31, the power density calculation unit 32, the correction value generation unit 33, and the power control unit 34 are provided by, for example, the controller 25 illustrated in FIG. When the controller 25 is realized by a processor system, the functions of the wavelength arrangement detector 31, the power density calculator 32, the correction value generator 33, and the power controller 34 are provided by the processor executing a control program. .

  The wavelength arrangement detector 31 detects the wavelength arrangement of the WDM signal output from the wavelength selective switch 12 (that is, the WDM signal input to the postamplifier 15) based on the OCM information output from the optical channel monitor 14. . Here, the optical channel monitor 14 detects the optical power of each wavelength channel of the WDM signal. Then, the wavelength arrangement detection unit 31 detects the wavelength arrangement of the WDM signal by specifying a wavelength channel whose detected optical power is higher than a predetermined threshold level. In the example shown in FIG. 7A, it is determined that the optical powers of the wavelength channels ch1, ch11 to ch19, and ch39 are higher than the threshold level, and the optical signals ch1, ch11 to ch19, and ch39 are multiplexed with the WDM signal. The In the example shown in FIG. 7B, it is determined that the optical powers of the wavelength channels ch1 and ch39 are higher than the threshold level, and the optical signals ch1 and ch39 are multiplexed with the WDM signal.

  The power density calculator 32 calculates the power density in the wavelength direction based on the wavelength arrangement detected by the wavelength arrangement detector 31. The power density is calculated for each optical signal that is multiplexed into the WDM signal. The power density represents the degree of congestion of optical signals arranged on the wavelength axis. Therefore, the power density is expressed by, for example, the number of blank channels (that is, the number of empty channels) between the optical signals arranged closest to each other. In this case, in the example shown in FIG. 7A, the power density of each optical signal is as follows.

The optical signal arranged closest to the optical signal ch1 is ch11. Here, nine empty channels (ch2 to ch10) exist between ch1 and ch11. Therefore, the power density of the optical signal ch1 is “9”. The optical signal arranged closest to the optical signal ch11 is ch12. Here, there is no empty channel between ch12 and ch12. Therefore, the power density of the optical signal ch11 is “0”. Therefore, in the example shown in FIG. 7A, the following power density is obtained for each optical signal multiplexed in the WDM signal.
ch1: 9
ch11-ch19: Zero ch39: 19

Similarly, in the example shown in FIG. 7B, the following power density is obtained for each optical signal multiplexed in the WDM signal.
ch1: 37
ch39: 37

  The correction value generation unit 33 generates a signal power correction value for each optical signal based on the power density of each optical signal. In this embodiment, the correction value generation unit 33 refers to the power density / correction value conversion table stored in the database 35 and acquires a signal power correction value for each optical signal. The database 35 is configured using, for example, the memory 26 shown in FIG.

  FIG. 8 shows an example of the power density / correction value conversion table. In this embodiment, the power density / correction value conversion table stores signal power correction values for the number of blank channels representing the power density. The power density / correction value conversion table is created in advance and stored in the database 35. The signal power correction value is relative to the power of the optical signal arranged in the wavelength region where the power density of the optical signal is low relative to the power of the optical signal arranged in the wavelength region where the power density of the optical signal is high. Is decided to be higher.

  In the power density / correction value conversion table shown in FIG. 8, “correction value = 0” means that the power of the optical signal does not change due to the correction. “Correction value = 3.5 dB” means that the power of the optical signal is increased by 3.5 dB by the correction.

  The correspondence between the power density and the signal power correction value is determined in advance by simulation or measurement. For example, the signal power correction value is determined with respect to the power density so that the width of the gain ripple of the EDFA in the steady state is reduced (that is, the gain is flattened with respect to the wavelength in the steady state). Alternatively, the signal power correction value may be determined with respect to the power density so that the change in the power of the optical signal remaining in the WDM signal becomes small when the wavelength arrangement of the WDM signal changes.

In the example shown in FIG. 7A, the following signal power correction value is obtained for each optical signal multiplexed in the WDM signal.
ch1: + 3.5dB
ch11-ch19: 0dB
ch39: + 3.5dB

In the example shown in FIG. 7B, the following signal power correction value is obtained for each optical signal multiplexed in the WDM signal.
ch1: + 3.5dB
ch39: + 3.5dB

  The power control unit 34 generates a power control signal for controlling the variable optical attenuator 23 so that the WDM signal has a specified wavelength characteristic. In this embodiment, the power control unit 34 generates a power control signal for equalizing the WDM signal output from the wavelength selective switch 12 (that is, the WDM signal input to the post amplifier 15). At this time, the power control unit 34 uses, for example, OCM information generated by the optical channel monitor 14 to make the power of the plurality of optical signals multiplexed in the WDM signal substantially the same. Generate a control signal. The power control signal includes an instruction for individually adjusting the power of a plurality of optical signals multiplexed in the WDM signal. This instruction represents the attenuation amount of the corresponding variable optical attenuator 23, for example.

  Further, the power control unit 34 corrects the power control signal generated as described above with the signal power correction value generated by the correction value generation unit 33. At this time, for example, the power control unit 34 adds the corresponding signal power correction value generated by the correction value generation unit 33 to the power control signal generated for each optical signal. However, when the power control signal represents the attenuation amount of the variable optical attenuator 23, the power control unit 34 corresponds to the corresponding value generated by the correction value generation unit 33 from the power control signal generated for each optical signal. The power control signal may be corrected by subtracting the signal power correction value.

For example, in the embodiment shown in FIG. 7A, it is assumed that the following attenuation is calculated as the power control signal in order to equalize the power of the optical signals ch1, ch11 to ch19, and ch39.
ch1: 4.5dB
ch11: 4.3dB
ch12: 4.3dB
...
ch19: 4.2dB
ch39: 4.1dB

Further, it is assumed that the signal power correction value described above is obtained. Then, the attenuation amount of each optical signal is corrected to the following value.
ch1: 1.0dB (= 4.5dB-3.5dB)
ch11: 4.3dB (= 4.3dB-0dB)
ch12: 4.3dB (= 4.3dB-0dB)
...
ch19: 4.2dB (= 4.2dB-0dB)
ch39: 0.6dB (= 4.1dB-3.5dB)

  The power control unit 34 applies the power control signal corrected for each optical signal to the corresponding variable optical attenuator 23. Then, the variable optical attenuator 23 adjusts the power of the optical signal according to the corrected power control signal. In the embodiment described above, the optical signal ch1 is attenuated by 1.0 dB, and the optical signal ch11 is attenuated by 4.3 dB. Thus, the attenuation amount of the variable optical attenuator 23 is corrected so that the power of the optical signals ch1 and ch39 is relatively higher than that of the optical signals ch11 to ch19.

  The multiplexer 24 of the wavelength selective switch 12 multiplexes the output signal of the variable optical attenuator 23 to generate a WDM signal. The post amplifier 15 amplifies the WDM signal output from the wavelength selective switch 12. In this example, the postamplifier 15 amplifies the WDM signal in the AGC mode. In AGC, the average gain of a WDM signal is held at a specified target value. Therefore, the optical transmission device 1 includes the AGC circuit 15a in order to realize AGC. The AGC circuit 15a controls the excitation power of the post amplifier 15 so that the ratio between the total optical power of the input WDM signal of the post amplifier 15 and the total optical power of the output WDM signal of the post amplifier 15 approaches the target value.

  FIG. 9 is a flowchart illustrating an example of processing of the controller 25. The process of this flowchart is executed, for example, when a change in the wavelength arrangement of the WDM signal is detected. The wavelength arrangement of the WDM signal changes when, for example, the optical signal is branched from the WDM signal, when the optical signal multiplexed with the WDM signal is stopped, or when the optical signal is added to the WDM signal. The controller 25 can detect a change in the wavelength arrangement of the WDM signal based on the OCM information generated by the optical channel monitor 14. The optical channel monitor 14 constantly monitors the power of each optical signal of the WDM signal.

  In S1, the controller 25 detects the power of each optical signal of the WDM signal based on the OCM information generated by the optical channel monitor 14. In S <b> 2, the power control unit 34 generates a power control signal for making the power of the plurality of optical signals substantially the same based on the power of each optical signal of the WDM signal. The power control signal represents, for example, an attenuation amount for each optical signal.

  In S3, the wavelength arrangement detector 31 detects the wavelength arrangement of the WDM signal based on the power of each optical signal. In S4, the power density calculation unit 32 calculates the power density in the wavelength direction based on the wavelength arrangement of the WDM signal detected by the wavelength arrangement detection unit 31. The power density is calculated for each optical signal. In this embodiment, the power density is expressed as an interval between adjacent optical signals. The interval between adjacent optical signals is represented by the number of blank channels in the above example, but may be represented by another index. For example, the interval between adjacent optical signals may be represented by a wavelength difference or a frequency difference. In S5, the correction value generation unit 33 generates a corresponding signal power correction value based on the power density of each optical signal.

  In S6, the power control unit 34 corrects the power control signal generated in S2 with the signal power correction value generated in S5. In S <b> 7, the controller 25 gives the corrected power control signal to the variable optical attenuator 23.

  Next, effects of the embodiment of the present invention will be described with reference to FIGS. That is, the effect by correcting the power control signal according to the wavelength arrangement of the WDM signal will be described.

  In the following, it is assumed that the wavelength arrangement of the WDM signal changes from the steady state shown in FIG. 7A to the transition state shown in FIG. That is, the WDM signal transitions from a state where the optical signals ch1, ch11 to ch19, ch39 are transmitted to a state where only the optical signals ch1, ch39 are transmitted. In FIG. 10B, FIG. 10C, FIG. 11B, and FIG. 11C, “11 waves” represents a steady state in which the optical signals ch1, ch11 to ch19, and ch39 are transmitted. , “2 waves” represents a transition state in which only the optical signals ch1 and ch39 are transmitted.

  First, the state of the WDM signal and the gain ripple when correction corresponding to the wavelength arrangement of the WDM signal is not performed will be described with reference to FIG. FIG. 10A shows the input optical power of the post amplifier 15. The power of the optical signal input to the post amplifier 15 is controlled according to the power control signal generated by the power control unit 34. Note that the input optical power of the post-amplifier 15 is substantially the same in the steady state and the transition state. However, the optical signals ch11 to ch19 do not exist in the transition state.

  FIG. 10B shows the output optical power of the preamplifier 11 of the optical transmission apparatus provided at the next node. That is, FIG. 10B shows a state after the optical signal shown in FIG. 10A is amplified by the post amplifier 15 and the preamplifier 11 of the next node. Also in FIG. 10B, the optical signals ch11 to ch19 do not exist in the transition state.

  The output optical powers of the optical signals ch1 and ch39 are different from each other in the steady state and the transition state. Specifically, when the optical signals ch11 to ch19 stop in a steady state, the output optical power of the optical signals ch1 and ch39 decreases. In particular, the optical power of the optical signal ch1 is greatly reduced when the optical signals ch11 to ch19 are stopped. The reason for this is that the gain ripple of the EDFA changes according to the change in the wavelength arrangement of the WDM signal.

  FIG. 10C shows a change in gain ripple in one OMS. As shown in FIG. 5, the OMS includes a postamplifier 15 and a preamplifier 11 at the next node. Here, when the optical signals ch11 to ch19 are stopped in the steady state, the gain of the EDFA changes from the state shown in FIG. 3C to the state shown in FIG. As a result, in this embodiment, the gain of the optical signal ch1 is reduced by about 2.0 dB, and the gain of the optical signal ch39 is reduced by about 0.2 dB. In this case, when the optical signals ch11 to ch19 are stopped, the optical power of the optical signal ch1 may be temporarily reduced by about 2.0 dB, and the optical power of the optical signal ch39 is temporarily reduced by about 0.2 dB. There is a fear.

  FIG. 11 shows the state of the WDM signal and the gain ripple when correction according to the first embodiment is performed. Similar to the example shown in FIG. 10, a steady state in which optical signals ch1, ch11 to ch19, and ch39 are arranged and a transition state in which only optical signals ch1 and ch39 are arranged will be described.

  FIG. 11A shows the input optical power of the post amplifier 15. Here, in the first embodiment, the power control signal is corrected with a signal power correction value generated according to the power density. That is, the power control signal is corrected so that the power of the optical signal arranged in the wavelength region with low power density is relatively higher than that of the optical signal arranged in the wavelength region with high power density. The In this embodiment, the power of each optical signal is corrected according to the conversion table shown in FIG. That is, a signal power correction value = 3.5 dB is generated for each of the optical signals ch1 and ch39. Therefore, compared with the state shown in FIG. 10A, the power of the optical signals ch1 and ch39 in the steady state is controlled to be higher by 3.5 dB by the correction. On the other hand, a signal power correction value = zero is generated for each of the optical signals ch11 to ch19. Therefore, the powers of the optical signals ch11 to ch19 in the steady state are substantially the same in FIGS. 10 (a) and 11 (a).

  Here, when the power of the optical signal increases, the spectrum “hole” of the spectral hole burning phenomenon becomes deeper. That is, when the power of the optical signals ch1 and ch39 is increased by correcting the power control signal, the gain in the wavelength region of the optical signals ch1 and ch39 is decreased. For example, the gain ripple of the EDFA when the power control signal is not corrected is represented by a solid line in FIG. The solid curve shown in FIG. 12 is the same as the gain characteristic shown in FIG. On the other hand, when the power control signal is corrected as described above, the gain in the wavelength region in which the optical signals ch1 and ch39 are disposed is relatively lowered as compared with other wavelength regions. As a result, as shown by a broken line in FIG. 12, the width of the gain ripple of the EDFA becomes small (or “hole” becomes shallow). That is, by correcting the power control signal, the gain of the EDFA with respect to the wavelength is flattened.

  FIG. 11B shows the output optical power of the preamplifier 11 of the optical transmission apparatus provided at the next node. As shown in FIG. 11B, even when the power control signal is corrected, when the optical signals ch11 to ch19 stop, the output optical power of the optical signals ch1 and ch39 decreases. However, when the power control signal is corrected, the change in the output optical power of the optical signals ch1 and ch39 due to the stop of the optical signals ch11 to ch19 is small compared to when the power control signal is not corrected. That is, in the case where the power control signal is not corrected, as shown in FIG. 10B, the power of the optical signal ch1 is reduced by about 2.0 dB due to the stop of the optical signals ch11 to ch19. On the other hand, in the case where the power control signal is corrected, as shown in FIG. 11B, the power of the optical signal ch1 is reduced by about 1.3 dB due to the stop of the optical signals ch11 to ch19.

  FIG. 11C shows a change in gain ripple in one OMS. In this example, when the optical signals ch11 to ch19 are stopped in a steady state, the gain of the optical signal ch1 is reduced by about 1.3 dB and the gain of the optical signal ch39 is reduced by about 0.2 dB. Thus, when the power control signal is corrected, the change in the gain ripple due to the change in the wavelength arrangement of the WDM signal is smaller than in the case where the power control signal is not corrected.

  As described above, when the wavelength arrangement of the WDM signal changes, the gain ripple of the EDFA (here, the postamplifier 15 and the preamplifier 11 of the next node) changes. Here, when the WDM signal is transmitted via a plurality of relay nodes, the change of the gain ripple is accumulated. FIG. 13 shows the change in accumulated gain ripple with respect to the number of relay nodes. In this embodiment, as the number of relay nodes increases, the change in the accumulated gain ripple increases. Note that the number of relay nodes corresponds to the number of OMS through which a WDM signal passes.

  As shown in FIG. 13, in the optical transmission system in which the power control signal is corrected in each node, compared to the optical transmission system in which the power control signal is not corrected in each node, the accumulated gain ripple caused by the change in wavelength arrangement The change of becomes smaller. Therefore, in the optical transmission system in which the power control signal is corrected, the decrease in power of the optical signals ch1 and ch39 (particularly ch1) due to the stop of the optical signals ch11 to ch19 is small.

  As an example, it is assumed that the number of hops of a WDM signal is 15 nodes. In this case, in the optical transmission system in which the power control signal is not corrected, when the optical signals ch11 to ch19 are stopped, the power of the optical signal ch1 at the receiving node temporarily decreases by about 14 dB. On the other hand, in the optical transmission system in which the power control signal is corrected, when the optical signals ch11 to ch19 are stopped, the power of the optical signal ch1 at the receiving node temporarily decreases by about 10 dB. That is, the received light power is improved by 4 dB by correcting the power control signal based on the wavelength arrangement of the WDM signal.

  Thus, according to the first embodiment, the amount of change in the power of the residual optical signal due to the change in the wavelength arrangement of the WDM signal is suppressed. Therefore, signal errors in the optical receiver are suppressed.

<Second Embodiment>
FIG. 14 illustrates an example of an optical transmission system according to the second embodiment. In the second embodiment, the power of each optical signal multiplexed on the WDM signal is controlled based on the wavelength arrangement of the WDM signal detected at the next node.

  At the next node, the preamplifier 11 amplifies the WDM signal transmitted through the transmission line fiber 2. The optical splitter 41 branches the WDM signal amplified by the preamplifier 11 and guides it to an optical channel monitor (OCM) 42. The optical channel monitor 42 detects the power of each optical signal multiplexed in the WDM signal amplified by the preamplifier 11.

  The wavelength arrangement detector 31, the power density calculator 32, the correction value generator 33, and the database 35 are substantially the same in the first and second embodiments. However, the wavelength arrangement detection unit 31 detects the wavelength arrangement of the WDM signal amplified by the preamplifier 11 based on the OCM information generated by the optical channel monitor 42. The power density calculation unit 32 calculates the power density in the wavelength direction based on the wavelength arrangement detected by the wavelength arrangement detection unit 31. The correction value generation unit 33 generates a signal power correction value for each optical signal based on the power density of each optical signal. At this time, the correction value generation unit 33 can obtain the signal power correction value for each optical signal with reference to the power density / correction value conversion table stored in the database 35. The functions of the wavelength arrangement detection unit 31, the power density calculation unit 32, and the correction value generation unit 33 are realized by the controller 25 provided in the next node.

  The controller 25 of the next node transmits the signal power correction value generated as described above to the upstream node. The controller 25 also transmits OCM information generated by the optical channel monitor 42 to the upstream node. This OCM information represents the power of each optical signal multiplexed in the WDM signal output from the preamplifier 11 of the next node.

  The operation of the power control unit 34 is substantially the same in the first and second embodiments. However, in the second embodiment, the power control unit 34 generates a power control signal for controlling the variable optical attenuator 23 based on the OCM information received from the next node. At this time, the power control unit 34 generates a power control signal that equalizes the power of the optical signal multiplexed in the WDM signal output from the preamplifier 11 of the next node. Then, the power control unit 34 corrects the generated power control signal with a signal power correction value received from the next node.

<Third Embodiment>
FIG. 15 illustrates an example of an optical transmission system according to the third embodiment. The third embodiment corresponds to a combination of the first embodiment and the second embodiment. That is, in the third embodiment, the first OMC information is generated for the WDM signal input to the postamplifier 15 using the optical channel monitor 14, and the next node's optical channel monitor 42 is used to generate the next node's optical channel monitor 42. Second OMC information is generated for the WDM signal output from the preamplifier 11. Then, the wavelength arrangement of the WDM signal is detected using one or both of the first OCM information and the second OSM information, and a signal power correction value is generated. In the third embodiment, each optical transmission is performed so that the wavelength characteristic of the WDM signal output from the postamplifier 15 and the wavelength characteristic of the WDM signal output from the preamplifier 11 of the next node are opposite to each other. An apparatus may be configured.

<Fourth Embodiment>
FIG. 16 illustrates an example of an optical transmission system according to the fourth embodiment. In the optical transmission system of the fourth embodiment, an inline amplifier 51 is provided between nodes. That is, the WDM signal output from the optical transmission apparatus 1 is amplified by one or a plurality of in-line amplifiers 51 and transmitted to the next node.

<Fifth Embodiment>
In the above-described embodiment, the optical transmission device 1 (1A, 1B) including the ROADM is provided in each node, but the optical transmission system according to the present invention is not limited to this configuration. For example, some nodes in the optical transmission system may be provided with an optical transmission apparatus that does not include the ROADM.

  FIG. 17 shows an example of an optical transmission system according to the fifth embodiment. In the embodiment shown in FIG. 17, an optical transmission device 3 not including ROADM is provided between optical transmission devices 1A and 1B including ROADM. The configuration of the optical transmission device 3 is similar to the optical transmission devices 1A and 1B. However, the optical transmission device 3 does not include the ROADM 22. That is, the optical transmission device 3 includes a wavelength blocker (WB: Wavelength Blocker) 61 instead of the wavelength selective switch (WSS) 22. The wavelength blocker 61 does not have a function of branching and inserting an optical signal, but has a function of adjusting the power of each optical signal.

  The operation of adjusting the power of each optical signal of the WDM signal in the optical transmission device 3 is substantially the same as that of the optical transmission devices 1A and 1B. That is, in the optical transmission device 3, the optical channel monitor 14 detects the power of each optical signal of the WDM signal output from the wavelength blocker 61. Then, based on the optical channel monitor 14, the controller 25 executes processing for generating a power control signal, processing for generating a signal power correction value, processing for correcting the power control signal with the signal power correction value, and the like.

  In the first to fourth embodiments, correction for suppressing gain ripple is performed on each OMS (Optical Multiplex Section). That is, correction for suppressing the gain ripple is performed for each ROADM. On the other hand, in the fifth embodiment, correction for suppressing the gain ripple is performed even in the section where the ROADM is not provided. In the example illustrated in FIG. 17, for example, correction for suppressing the gain ripple is performed between the optical transmission device 1 </ b> A and the optical transmission device 3, and the gain is increased between the optical transmission device 3 and the optical transmission device 1 </ b> B. Correction for suppressing ripples is performed.

<Other embodiments>
Various variations are conceivable as a method for generating the signal power correction value. For example, “correction value = predetermined minimum value” is given to an optical signal arranged in a wavelength region having the highest power density, and “correction value = relative value to minimum value” is given to another optical signal. You may do it. The minimum value is, for example, 0 dB. When the WDM signal shown in FIG. 7A is transmitted, optical signals ch11 to ch19 are arranged in the wavelength region with the highest power density. In this case, “correction value = 0 dB” is given to the optical signals ch11 to ch19. The optical signals ch1 and ch39 are given correction values higher by 3.5 dB with respect to the optical signals ch11 to ch19. That is, “correction value = 3.5 dB” is given to the optical signals ch1 and ch39.

  The signal power correction value may be a negative value. When the signal power correction value is negative, in this embodiment, control for reducing the power of the optical signal is performed. As an example, if “correction value = 0 dB” is given to “number of blank channels = 3” in the table shown in FIG. 8, “correction value = −1.5 dB” for “number of blank channels = 1 or less”. ”And“ correction value = 2.0 dB ”is given to“ the number of blank channels = 9 or more ”. The correction value may be limited to a positive value.

If the power of the optical signal is too high, non-linear degradation (self-phase modulation, cross-phase modulation, etc.) may occur in the transmission line fiber. On the other hand, if the power of the optical signal is too low, the optical signal-to-noise ratio may be degraded. Therefore, the signal power correction value may be determined between a predetermined upper limit and lower limit. For example, when the upper limit is 3 dB and the lower limit is −1 dB, the following correction value is generated for each optical signal of the WDM signal shown in FIG.
ch1: + 3.0dB
ch11-ch19: 0dB
ch39: + 3.0dB

You may determine each correction value so that the average of the correction value given to each optical signal may become zero. As an example, when the upper limit is 3 dB, the lower limit is −1 dB, and the average of the correction values is zero, the following correction values are generated for each optical signal of the WDM signal shown in FIG. .
ch1: + 2.7dB
ch11-ch19: -0.6dB
ch39: + 2.7dB

In the above-described embodiment, the correction value generation unit 33 acquires the correction value corresponding to the power density using the table shown in FIG. 8, but the present invention is not limited to this configuration. That is, the correction value generation unit 33 may calculate a correction value based on the power density. In this case, constants and the like used in the calculation formula are stored in the memory 26, for example. As an example, the correction value may be calculated by the following equation. B represents the number of blank channels.
Correction value = 5 * log10 [(B + 1) / 2]

  In the above-described embodiment, the power density corresponding to each optical signal is represented by the interval to the adjacent optical signal, but the present invention is not limited to this method. For example, when calculating the power density of one optical signal of interest in the WDM signal, the “wavelength characteristic acting on the optical signal of interest due to all other optical signals in the WDM signal” is determined. Also good. In this case, the power density is calculated by convolving and integrating the signal wavelength arrangement with this wavelength characteristic. The wavelength characteristics used in the convolution integration may simply be a normal distribution (σ: about 2 nm).

1 (1A, 1B) Optical transmission device 2 Transmission line fiber 11 Preamplifier 12 Wavelength selective switch (WSS)
14, 42 Optical channel monitor (OCM)
15 Postamplifier 22 Optical add / drop multiplexer (ROADM)
23 Variable Optical Attenuator (VOA)
25 Controller 26 Memory 31 Wavelength Arrangement Detection Unit 32 Power Density Calculation Unit 33 Correction Value Generation Unit 34 Power Control Unit 35 Database

Claims (8)

An optical transmission system including a first optical transmission device and a second optical transmission device that receives a wavelength multiplexed optical signal transmitted from the first optical transmission device,
A wavelength arrangement detector that is provided in the first optical transmission apparatus or the second optical transmission apparatus and detects a wavelength arrangement representing an arrangement of optical signals multiplexed in the wavelength-multiplexed optical signal;
A power adjuster that is provided in the first optical transmission device and adjusts the power of the optical signal multiplexed in the wavelength-multiplexed optical signal;
An optical amplifier that is provided in the first optical transmission device and amplifies a wavelength-multiplexed optical signal output from the power adjuster;
A power control unit for generating a power control signal for controlling the power adjuster so that the wavelength multiplexed optical signal has a specified wavelength characteristic;
A correction value generating unit that generates a correction value for correcting the power control signal based on the wavelength arrangement, and
The power control unit corrects the power control signal with the correction value;
The power adjuster adjusts the power of the optical signal multiplexed on the wavelength-multiplexed optical signal according to the corrected power control signal. The power control unit is provided in the first optical transmission device,
The wavelength arrangement detection unit and the correction value generation unit are provided in the second optical transmission device,
The optical transmission system according to claim 1, wherein the power control unit receives the correction value from the second optical transmission device and corrects the power control signal with the received correction value. The second optical transmission device includes an optical preamplifier that amplifies a wavelength multiplexed optical signal received from the first optical transmission device,
The optical transmission system according to claim 2, wherein the wavelength arrangement detection unit detects a wavelength arrangement of the wavelength multiplexed optical signal amplified by the optical preamplifier. A wavelength arrangement detection unit for detecting a wavelength arrangement representing an arrangement of optical signals multiplexed in the wavelength division multiplexed optical signal;
A power adjuster for adjusting the power of the optical signal multiplexed in the wavelength-multiplexed optical signal;
An optical amplifier that amplifies the wavelength-multiplexed optical signal output from the power regulator;
A power control unit for generating a power control signal for controlling the power adjuster so that the wavelength multiplexed optical signal has a specified wavelength characteristic;
A correction value generating unit that generates a correction value for correcting the power control signal based on the wavelength arrangement, and
The power control unit corrects the power control signal with the correction value;
The power adjuster adjusts the power of the optical signal multiplexed on the wavelength-multiplexed optical signal according to the corrected power control signal. A power density calculator that calculates a power density at a wavelength at which an optical signal multiplexed with the wavelength-multiplexed optical signal is arranged based on the wavelength arrangement;
The optical transmission device according to claim 4, wherein the correction value generation unit generates the correction value based on the power density. The optical transmission according to claim 5, wherein the power density calculation unit calculates a power density based on an interval to an adjacent optical signal for each optical signal multiplexed in the wavelength-multiplexed optical signal. apparatus. The correction value generator generates a correction value for relatively increasing the power of an optical signal arranged in a wavelength region with a low power density relative to the power of an optical signal arranged in a wavelength region with a high power density. The optical transmission system according to claim 5, wherein the optical transmission system is generated. A memory for storing information representing a correspondence relationship between the power density and the correction value;
The power density calculation unit calculates a power density for each optical signal multiplexed in the wavelength multiplexed optical signal,
The correction value generation unit generates a correction value corresponding to a power density with reference to information stored in the memory for each optical signal multiplexed in the wavelength multiplexed optical signal. The optical transmission system according to claim 5.